1. Field of the Invention
The present invention relates to an ink-eject printer for printing images by changing liquid ink into droplets and ejecting these on objects to be printed, and in particular to an ink-eject printer for squirting ink droplets by pressure of acoustic waves emitted from piezoelectric elements and ejecting these ink droplets on the objects.
2. Description of the Related Art
A device for printing images by controlling liquid ink into small particles called droplets and ejecting these on a printing medium so as to form pixels has been practically used as an ink-eject printer.
Generally, in a device using an ink-eject printing system, local ink concentration easily occurs due to solvent evaporation or volatilization, causing clogging of an individual thin nozzle corresponding to resolution. For instance, in a system using vapor pressure for forming ink ejects, deposition of insolubles produced by thermal and chemical reaction with ink occurs, and in a system using pressure of a piezoelectric element, clogging of the nozzle occurs because of a complex ink flow passage, etc. Usually, in a serial scanning type head using several tens to hundred and several tens of nozzles, some treatments are given in order to reduce a frequency of clogging. However, in the case of a line scanning type head for which several thousands of nozzles are necessary, a frequency of clogging is high and thus reliability of an ink-eject printer declines.
In a conventional ink-eject printer, there are some problems to be solved when resolution is to be improved. For instance, in a device using vapor pressure, it is difficult to produce ink droplets having a particle diameter of 20 .mu.m or less (equivalent to a printing dot having a diameter of about 50-odd .mu.m). Also, in a device using pressure produced by a piezoelectric element, because of a complex structure of a printing head a problem occurs in terms of a technique for processing this printing head and thus it is difficult to manufacture one for realizing high resolution.
In recent years, there has been presented a system for ejecting ink from ink liquid surface by using pressure of acoustic waves generated by means of a piezoelectric element composed of a thin film piezoelectric element. This system is called a nozzleless system, in which a nozzle for each individual dot or any partitions for ink flow passages are not necessary. When this system is applied to the line scanning type head, clogging is prevented and restoration is rather easy even if clogging occurs. Moreover, this is suited for obtaining high resolution, since ink droplets of very small diameters can be stably produced and ejected.
In order to realize printing of high image quality in such an ink-eject printer using acoustic waves, it is necessary to increase gray scales in addition to improvement of resolution. An increase in gray scales means that it is possible to form desired gray-level of half tone pixels.
As described above, the improvement of resolution can be realized by forming ink droplets in fine particles. For the increase in gray scales, there has been presented a method for realizing a plurality of gray scales by, for instance controlling sizes of ink droplets or the number thereof in multi-levels. As an example, in Japanese Patent Application Kokai Publication No. 63-16645 by Xerox Co. Ltd., there have been disclosed two methods: the first is a method for squirting and ejecting a plurality of ink droplets by repeatedly driving a piezoelectric element of an ink-eject head for one pixel by a plural number of times and superpose the ink droplets on an object to be printed before the previously ejecting ink droplets are dried and the second is a method for controlling sizes of ink droplets by adjusting a drive signal applied to a piezoelectric element.
In the first method for performing printing per pixel by superpose a plurality of ink droplets, however, it is necessary to repeatedly drive the piezoelectric element by a plural number of times. As a result, when multi-level pixels are to be realized, it takes a long time to perform printing per a pixel and thus it is not suited to high-speed printing. In the second method for changing the sizes of ink droplets, an influence given on the sizes of ink droplets is relatively small and thus it is impossible to increase the number of gray scales.